Determining the role of statins in Parkinson's disease risk reduction and disease modification: A comprehensive meta‐analysis of 4 million participants' data

Abstract Background Many observational studies have examined the association between statins and the incidence of Parkinson's disease (PD) in high‐risk populations. On the other hand, clinical trials as well as other observational studies investigated the safety and efficacy of statins in slowing disease progression in PD patients. However, the evidence has been inconclusive in both questions. To that end, we conducted this systematic review and meta‐analysis to synthesize evidence on the role of statins in decreasing the risk of PD among high‐risk populations and as a possible disease‐modifying agent for patients with PD. Methods A comprehensive literature search of electronic databases including PubMed, Scopus, Cochrane, and Web of Science has been performed. Relevant studies were chosen and data were extracted and analyzed using RevMan software version 5.4.1. Results Twenty‐five studies (14 cohort, 9 case–control, and 2 randomized controlled trials) have been included in the present systematic review. Of them, 21 studies reported the association between statins and PD risk. Statins were found to significantly reduce the risk of developing PD (pooled RR 0.86, 95% CI [0.77–0.95], p < 0.005). Four studies investigated statins as a disease‐modifying agent. The pooled mean difference (MD) in the UPDRS‐III from baseline to endpoint did not differ significantly between the statin and control groups (MD −1.34 points, 95% CI [−3.81 to 1.14], p = 0.29). Conclusion Although epidemiological observational studies showed that statin use was associated with a reduced risk of PD, current evidence is insufficient to support the role of statins in slowing the progression of PD. These findings are limited by the fact that most of the included studies are observational studies which carry a high risk of confounding bias which highlights the need for future well‐designed RCTs.

Parkinson's disease (PD) is a chronic, progressive neurodegenerative disorder affecting approximately 1% of the population over 60. 1 It is characterized by the loss of dopamine-producing neurons in the substantia nigra region of the brain, causing motor and non-motor symptoms, such as tremors, rigidity, bradykinesia, postural instability, cognitive impairment, and depression. 2 The exact cause of PD is unknown; however, neuroinflammation and microglial activation are among the key pathological processes involved in the disease. 3spite the extensive research over the past decades, current treatment options for PD are mainly symptomatic with no drugs have been found to prevent, slow down, or cure PD.
A simple and rapid method of therapeutic development is to screen existing drugs for potential benefits for the underlying disease.Statins are a class of medications commonly used in lowering cholesterol levels.Recent studies have shown that statins have antiinflammatory properties that may play a role in the progression and treatment of PD. 4 This was further corroborated by some epidemiological studies reporting a significant association between statins and the reduced risk of PD.However, this evidence is conflicting and not conclusive.For example, a prospective study in 2015 showed that statin use was associated with significantly higher PD risk (OD 2.39, 95% CI [1.11-5.13]) 5 while another study in 2016 found that statin use was linked to lower risk of PD (HR 0.65, 95% CI [0.57-0.74] in females and 0.60 95% CI [0.51-0.69] in males). 6bsequent clinical trials investigated the potential diseasemodification effects of statins in PD.A randomized controlled trial (RCT) in 2021 found a beneficial effect of lovastatin in reducing the mean UPDRS motor score compared to placebo (MD −3.18 vs. −0.50;p = 0.14). 7On the other hand, another RCT in 2022 found that simvastatin was associated with deterioration in the UPDRS III score compared with the placebo group (1.52 points). 80][11][12][13][14][15] However, their inclusion criteria were limited only to observational studies and the only outcome of interest was the risk of PD development in high-risk populations.
We conducted this systematic review and meta-analysis to synthesize evidence that answers two research questions: (1) is statin use associated with a lower risk of PD in high-risk populations?And (2) does statin supplementation slow the progress of PD compared to placebo?

| ME THODS
We followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement guidelines for reporting this systematic review and meta-analysis. 16The protocol of this study was prospectively registered in PROSPERO (CRD42023396879).

| Inclusion and exclusion criteria
When selecting studies for this review, eligibility criteria were defined using the participants, interventions, comparisons, outcomes, and study design (the PICOS model).
For the first research question, we included studies where (1) the population was patients with PD, (2) the intervention was statins, (3) the comparator was normal controls, (4) the outcome was the risk of PD occurrence, and (5) the study design was either cohort or case-control.
For the second research question, studies that meet the following criteria were included: studies where (1) the population was patients with PD, (2) the intervention was statins, (3) the comparator was normal controls, (4) the outcome was the changes in Unified Parkinson's Disease Rating Scale-Part III (UPDRS-III), and (5) the study design was RCT or observational study.
Studies that meet the following criteria were excluded: (1) basic science and preclinical laboratory studies that did not involve any clinical data, (2) publications that were case report, case series, or literature review, and (3) studies in languages other than the English language.

| Search strategy and keywords
A comprehensive internet search of electronic databases including PubMed, Scopus, Cochrane, and Web of Science has been performed until February 2023.A specific set of keywords and search strategies were used to ensure that the results were relevant.The keywords used were "statin(s)," "HMG-CoA reductase inhibitor(s)," "simvastatin," "atorvastatin," "pravastatin," "fluvastatin," "rosuvastatin," "lovastatin," "Parkinson's disease," and "PD." The keywords were combined with each other using the Boolean operators "AND" and "OR."Additional records were identified through previous systematic reviews and a manual search.All the role of statins in slowing the progression of PD.These findings are limited by the fact that most of the included studies are observational studies which carry a high risk of confounding bias which highlights the need for future well-designed RCTs.

K E Y W O R D S
disease risk, hydroxymethylglutaryl-coenzyme A reductase inhibitors, meta-analysis, Parkinson's disease, statins, symptom progression electronic records were exported to Rayyan software to conduct the screening process. 17

| Selection of studies
To ensure the selection of relevant studies in our research project, a two-phase screening process was conducted with the participation of all authors.During the first phase, two groups of authors independently assessed the eligibility of the title and abstract of each article, and this process was repeated twice to ensure accuracy.In the second phase, the full text of all articles that passed the first phase was examined to identify potentially included studies for our research project.

| Data extraction
A data extraction excel sheet that was accessible to all authors was created.All authors participated in data extraction.Extracted data for each study included: study ID (last name of first author and the publication year), country, study design, length of follow-up, the total number of participants, study population, number of PD patients, outcome measures, and key findings.

| Assessment of risk of bias in included studies
We used the Newcastle-Ottawa Scale (NOS) to assess the risk of bias in the included observational studies. 18The NOS is a tool specifically designed to evaluate the quality of observational studies and consists of eight items grouped into three domains.These domains include the selection of study participants (four points), comparability between groups (two points), and ascertainment of exposure (in case-control studies) or outcomes (in cohort studies) (three points).The maximum score of nine points indicates the least risk of bias.Based on the total score, we categorized the studies into low, medium, or high risk of bias, with scores of ≥9, 7-8, and <7, respectively.Additionally, we used the Cochrane risk of bias-2 (ROB 2) tool to assess the risk of bias in the included RCTs. 19This tool evaluates several aspects of the RCTs, including sequence creation, allocation concealment, blinding, inadequate outcome data, selective outcome reporting, and other potential sources of bias.

| Measures of drug effect and statistical analysis
We conducted this systematic review and meta-analysis to synthesize evidence that answers two research questions: (1) is statin use associated with lower risk of PD in high-risk populations?And (2) does statin supplementation slow the progress of the disease compared to placebo?
For the first research question, the primary outcome measure was PD risk.It is measured as odds ratio (OR), risk ratio (RR), or hazard ratio (HR).OR represents the odds of using statins in the past compared between the PD patients and non-PD controls.An RR greater than one indicates an increased risk of PD and an RR less than 1 indicates a decreased risk of PD.All effect sizes represented as RR, OR, and HR were included and treated the same because of the low incidence of PD. 20 For the second research question, the primary outcome measure was motor functions measured by the UPDRS-III score.This continuous outcome was expressed as the mean difference (MD) between the study groups in terms of the changes from the baseline.The MDs and their corresponding 95% CIs were pooled in the meta-analysis models.
All the effect sizes were pooled in a meta-analysis model using the RevMan software version 5.4.1.A random-effects metaanalysis model, using DerSimonian-Laird method, was employed to calculate the final pooled effect estimate.This model was chosen over the fixed-effects model to account for withing-study and between-study variability.Following the main analysis, we preplanned for subgroup analyses according to study design, location, adjustment for age, adjustment for sex, adjustment for smoking, and statin type.
In case of missing data, we contacted the corresponding authors to request them.Second, in case of nonresponse, for studies that reported only pretreatment and posttreatment means, the mean change was calculated by subtracting baseline (pretreatment) from final (posttreatment) values and standard deviations of change were imputed according to Cochrane recommendations using the equation below with a conservative correlation coefficient (r = 0.5). 19her missing data were imputed using the available data, for example, confidence interval, p-value, t-value, and standard error, according to Cochrane Handbook recommendations.For the statistical analysis, we used the Review Manager version 5.4.1 for Windows.

| Assessment of heterogeneity
First, heterogeneity was inspected visually in the forest plots by looking for the absence of any common overlapping point among all the pooled 95% confidence intervals.Statistical heterogeneity was assessed using the chi-squared test, which tests the chi-squared distribution of the Cochrane Q test.We used the I-squared test to quantify the amount of variation among studies caused by factors other than chance.I-squared values were interpreted as low (25%), moderate (50%), and high (75%). 19

| Publication bias
Publication bias was assessed by visual inspection of funnel plots of the meta-analysis models.The existence of asymmetrical distribution of the studies around the pooled effect estimate indicates the possibility of publication bias.According to Sterne et al., 21 funnel plot asymmetry should not be used as a tool to assess the publication bias unless there are at least 10 studies in the meta-analysis.
If case of less than 10 studies, the power of the test is too low to distinguish chance from real asymmetry.

| Search results
We identified 2519 references through computerized and manual searches, removed 629 duplicates, and screened the titles and abstracts of the remaining 1890 records.We excluded 1808 records after reviewing the titles and abstracts because they did not meet the prespecified inclusion criteria.The full texts of the remaining 82 studies were obtained and assessed again according to the eligibility criteria.This time, we excluded 57 studies because they were either animal experiments, in vitro experiments, review articles, case reports, comments, replies, irrelevant studies, or meeting abstracts, leaving 82 records.Finally, 25 studies including 23 observational studies and 2 RCTs fulfilled our inclusion criteria and were included in the meta-analyses (Figure 1).This includes 21 studies addressing the first research question (Does statin reduce the risk of PD in high-risk populations?) and four studies addressing the second research question (Among patients with PD, does statin supplementation slow the progress of PD?).

| Study characteristics
The present systematic review included 25 studies with 17 years as a span of publication time of the articles starting from 2006 and ending in 2023.Among the included studies, 23 were observational (14 cohort and 9 case-control), and 2 were RCTs. 7,8Nine of the studies were conducted in the United States, 5,22-29 three in the United Kingdom, 8,30,31 two in France, 32,33 two in Israel, 34,35 two in China, 6,36 two in Taiwan, 7,37 one in South Korea, 38 one in Italy, 39 one in the Netherlands, 40 one in Denmark, 41 and one in Canada. 42The total number of participants included in all studies was 4,071,794, includ- was detected by reviewing medical records in 6 articles, databases in 13 studies, and self-reporting in 4 studies.

| Methodological quality
ِ After applying NOS to the 23 included studies, we categorized the studies into high, medium, or low quality, with scores of 9, 7-8, and <7, respectively.42 The NOS score of each study is shown in Table 1.

| The association between statin use and PD Risk
The pooled RR from 21 studies showed a significant association between statin use and a reduced risk of PD.Individuals with a history of statin use had lower risk of developing PD (RR 0.86, 95% CI (0.77-0.95), p < 0.005) (Figure 2).The random-effect model was used because of the significant heterogeneity among studies (χ 2 p < 0.00001; I 2 = 87%).

| Sensitivity analysis for PD risk
We conducted a sensitivity analysis for the PD risk outcome in mul- tiple scenarios by excluding one study in each scenario and repeated this process 21 times.This method is used to investigate whether a particular study is accounting for the underlying high heterogeneity.
We found that heterogeneity persisted in every scenario meaning that heterogeneity cannot be attributed to a single study but larger variations between the studies.

| Subgroup analysis for PD risk
We tried to explain the heterogeneity by conducting subgroup analysis according to six different factors (namely, study design, location, adjustment for age, adjustment for sex, adjustment for smoking, and statin type) as presented in Table 2.This method failed to explain the heterogeneity as the heterogeneity persisted in most of the study groups except for the following groups: The studies conducted in Europe (χ 2 p = 0.47, RR 0.87, 95% CI [0.81-0.93],p < 0.001) and the studies that did not adjust for age (χ 2 p = 0.66, RR 0.87, 95% CI [0.82-0.93],p < 0.001).This means that the heterogeneity could not be completely explained by variation in any of these factors but likely by deeper differences in the study methodology.
The effect estimate continued to show statistical significance in cohort studies, the studies conducted in Europe, the studies conducted in Asia, the studies that adjusted for age, the studies that adjusted for sex and the studies that did not, and the studies that used simvastatin and atorvastatin.In the remaining groups, the result was not statistically significant.

| Publication bias for PD risk
In accordance with the prespecified criteria described above in the methods section (see 2.8.publication bias), the number of the included studies in the meta-analysis model of the first research question (>10) allowed for publication bias assessment using the funnel plot.The visual interpretation of the funnel plot (Figure 3) showed a symmetrical scatter of points on both sides of the mean effect size.
This suggested a wide distribution of the study variabilities.

| The pooled change in UPDRS III
The pooled meta-analysis did not find a significant difference in UPDRS III scores between the statins and control groups (MD −1.34 points, 95% CI [−3.81 to 1.14], p = 0.29, Figure 4).This analysis was based on four studies (two observational and two RCTs) with a total of 464 participants (statins: n = 211 patients and placebo: n = 253 patients).However, the studies included in the analysis were not homogeneous as indicated by the significant chi-squared p-value and I-squared value (p = 0.02, I 2 = 69%).

F I G U R E 2
Main meta-analysis results for the first research question (the association between statins and PD risk).

| Subgroup analysis
We run a subgroup analysis according to the study design.This difference could be explained by the higher baseline UPDRS-III score compared to the other studies.

| Publication bias
Based on the criterion descripted above in the Methods section, the assessment of publication bias for the second research question was not reliable because the number of included studies was insufficient. 43Therefore, we did not run the publication bias for this meta-analysis model.

| GRADE assessment of the certainty in evidence
Based on GRADE assessment, the quality of evidence on the changes in UPDRS-III score (MD −1.34 points with 95% CI [−3.81 to 1.14]) was evaluated as "low."The quality of evidence is limited by the observational design of two studies, which lacks random allocation.There is also a significant heterogeneity in the pooled estimate of UPDRS-III score (Table 3).Therefore, the evidence is downgraded by two levels from the initial evaluation "moderate." The "low" assessment of evidence means that the true effect might be markedly different from the estimated effect.Therefore, future studies are needed to increase our confidence in the estimate.

| Summary of the key findings
Our study revealed that individuals with a history of statin use have less risk of developing PD compared to those who do not have a history of statin use.However, statin supplementation for PD patients did not slow PD progression or improve motor functions.Our metaanalysis does not support the use of statin for PD patients.

| Explanation of the study findings
For the first research question, the subgroup analysis according to individual statins showed statistically significant results for simv- and lovastatin).Lipophilicity is an important characteristic of drugs that enables them to cross the blood-brain barrier (BBB) and allows their central nervous system action.Lipophilic statins like simvastatin and atorvastatin could cross the BBB, which can help explain our findings.When comparing simvastatin to atorvastatin, our subgroup analysis shows that simvastatin was more protective than atorvastatin which confirms the results of previous studies that simvastatin is superior to atorvastatin at crossing the BBB. 44 the second research question, the only study that exclusively used simvastatin 8 was the only study that reported an increased,

| Previous systematic reviews
Our results were consistent with a previous study conducted by Wu et al., 13  risk.We investigated the potential of statins as a disease-modifying agent to hinder motor symptom progression in patients with PD.We included four additional studies for this purpose.

| Strengths and limitations
Regarding strengths, our study is the most comprehensive one on this topic ( We investigated different statins, including simvastatin, lovastatin, atorvastatin, and pravastatin, as a risk factor for developing PD and a disease-modifying agent to improve the clinical outcomes in PD patients.Our meta-analysis included more than 4 million participants from around the world.We performed broad subgroup and sensitivity analyses that would impact clinical decisionmaking.Additionally, we think our study opens the door for more interventional studies considering the therapeutic effects of different statins for PD.
Limitations in our study included the significant heterogeneity among the studies in the main and subgroup analyses, which we tried to resolve by performing sensitivity and subgroup analyses.
The variability between the study designs, durations, participants' characteristics, and outcome measures justifies this heterogeneity.
Second, our study could not conclude the effect of different doses of statins on the outcomes of interest because of inadequate data.
Third, most of the included studies (n = 23) were observational studies.Therefore, there might be several potentially confounding factors to consider.
Finally, it should be mentioned that mixing data from both observational studies and RCTs in the same meta-analysis (the second research question in our review) is controversial as it may introduce bias to the results because of the inherent difference in observational and interventional studies.However, according to Borenstein et al., 45 observational studies and RCTs might be put together if they do not disagree with each other and are believed to address a common question.Moreover, we conducted subgroup analysis according to the study design.Both groups (observational group and RCT group) showed consistent overall effect size in the same direction which further justifies the combining of both designs.

| Authors' conclusion
Although epidemiological observational studies showed that statin use was associated with a reduced risk of PD occurrence, cur-

ing 35 ,
283 PD cases.Among the 23 observational studies, statin use F I G U R E 1 PRISMA flow diagram showing the details of the study selection process.
For the subgroup of cohort studies, the overall MD in UPDRS-III between statins and control did not favor either group (MD −2.26 points, 95% CI [−5.43 to 0.92], p = 0.16).Similarly, for the subgroup of RCTs, the overall MD in UPDRS-III between statins and placebo did not favor either group, with a nonsignificant MD of −0.36 (95% CI [−5.04 to 4.32], p = 0.88).

3. 5 . 3 |
Sensitivity analysisWe conducted a sensitivity analysis by excluding each individual study at a time.Excluding the study of Stevens 2022 8 from the metaanalysis model resolved the heterogeneity (χ 2 p = 0.26, I 2 = 26%) as presented in Figure 5.After removing this study, the overall MD between statin and placebo favored statins, with a statistically significant MD of −2.43 points (95% CI [−4.24 to −0.62], p = 0.009).

F I G U R E 3
Funnel plot for assessing publication bias for the first research question (the association between statins and PD risk).

| 9 of 12 MADY
Abbreviation: N/A, not applicable because of the small number of included studies (Egger et al).a Owing to the potential risk of bias in addition to the inclusion of non-RCT studies.bOwing to the significant heterogeneity in the effect estimate (I 2 = 69%).
rent evidence is insufficient to support the role of statins in slowing the progression of PD.These findings are limited by the fact that most of the included studies are observational which carry a high risk of confounding bias.Therefore, future well-designed RCTs are recommended.AUTH O R CO NTR I B UTI O N SStudy conceptualization and design: Ahmed Negida and AbdelrahmanMady.Protocol design: Abdelrahman Mady.Abstract screening on Rayyan, full-text screening and study selection, data extraction, and quality assessment: All authors participated in these steps.Data analysis: Asma Daoud, Abdelrahman Mady, Menna Marwan, and Moaz Elsayed.Writing introduction and conclusion: Abdulrahman Krayim and Maickel Abdelmeseh.Writing methods: Abdelrahman Mady and Moaz Elsayed.Writing results: Abdelrahman Mady, Asmaa Alnajjar, and Taleb Alsalloum.Writing discussion: Abdelrahman Mady, Maickel AbdelMeseh, Menna Marwan, Asmaa Alnajjar, Taleb, Mohamed Alaa, and Nagham Abdalla.Proofreading the manuscript: Ahmed Negida.Supervision and revision: Matthew J. Barrett, Ahmed Negida, and Brian Berman.A FFI LI ATI O N S Characteristics of the included studies.
The results of subgroup analyses based on various factors.
than decreased, rate of symptom progression of PD with which suggested that statins may be beneficial for reducing

Table 4
Comparison with previous systematic reviews.
).It includes observational and interventional TA B L E 4